54^ ERIK ZEUTHEN 



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Discussion 



Mazia : One point made by Zeuthen could be stressed because it would apply 

 to division in cells other than Tetrahymena. While the "division protein" may be 

 crucial, it needs not represent very much protein. His proof is that the absence of 

 availability of external nutritional sources for proteins makes little or no difference 

 to the division of his synchronized cell populations. It has also been observed in 

 studies on fission yeasts by Faed in Mitchison's laboratory that the cells may go 

 through a complete division cycle or two in the absence of external nitrogen 

 sources, producing small progeny. The "division protein" may be available in 

 small but adequate amounts, may be supplied by conversion of other proteins, or 

 may be made from the amino acid pool. The fact that it is crucial does not imply 

 that it represents a quantitatively important fraction of the protein synthesis taking 

 place during the growth-division cycle. 



Davis: Dr. Zeuthen, did I understand correctly that during this period of 

 growth without division DNA and RNA continued to be synthesized at normal 

 rates ? I wonder if your problem might be a little analogous to one observed with 

 bacteria, where under the influence of many inhibitory agents at border-line 

 concentrations the cells continue to grow and become tremendously elongated, 

 but do not divide. The limitation appears to be the completion of the septum which 

 leads to division. Could yours be a problem where the limiting factor is cell 

 membrane formation ? 



Zeuthen : We think we have put our fingers — very lightly — on a protein which 

 is specifically engaged in division but, as Dr. Alazia said, may be only a very small 

 part of the whole cell. What does this protein perform, where does it sit ? We do 

 not know. Our problem could be analogous to the situation mentioned by Dr. 

 Davis. We have incubated the washed cells with labelled amino acids. In radio- 

 autographs the label seems to sit everywhere. We may now try to fractionate for 

 cell walls ("pellicles"), nuclei, particles and so on, to see if the label is attached 

 predominantly to one of these organelles. Another possible approach would be that 

 we try to separate the proteins after previous incubation of the cells for short and 

 different times with labelled amino acids. One, or more, proteins might take the 

 label in excessive amounts. 



